Case 1
- Signal amplitude
- 2
- Noise amplitude
- 0.02
40.00dB
Open with these valuesSignal-to-Noise Ratio Calculator
40.00dB
Result: 40.00 dBFor amplitudes — volts, millivolts, converter counts — the ratio is 20 × log10(signal ÷ noise). A signal of 2 against noise of 0.02 is 100 times larger, which is 40 dB. Use 10 × log10 instead when both figures are powers in watts, not amplitudes.
Worked examples
Case 2
- Signal amplitude
- 10
- Noise amplitude
- 1
20.00dB
Open with these valuesCase 3
- Signal amplitude
- 1
- Noise amplitude
- 0.001
60.00dB
Open with these valuesHow it's calculated
SNR = 20 × log10(signal ÷ noise)
- StepEnter the signal amplitude and the noise amplitude, in one unit.
- StepDivide the signal by the noise to get the bare ratio.
- ResultTake the base-10 logarithm and multiply by 20 for decibels.
What this number means
This one uses 20 × log10, for amplitudes
Both fields expect amplitudes, which is why 2 against 0.02 gives 40 dB rather than 20. Powers in watts belong in 10 × log10, and putting them here doubles every figure.
Decibels are a logarithm, not a plain ratio
Each 20 dB stands for a factor of 10 in amplitude, and therefore 100 in power. So 40 dB says the signal is 100 times the noise, not 40 times.
The unit cancels, so use whichever you have
Volts, millivolts or raw converter counts all work, provided both fields carry the same one. The division removes the unit and leaves a bare decibel figure.
Below zero the noise is winning
At 0 dB signal and noise are equal, and anything negative means the noise is louder. Modern radio links still decode below 0 dB by spreading each bit over a longer time.
Commonly misread
40 dB means the signal is 40 times the noise.
It means 100 times in amplitude, since the scale is logarithmic. Already 20 dB is a factor of 10.
A negative result is an input error.
It only says the noise exceeds the signal, as 0.1 against 1 does at −20 dB. The formula covers that case.
20 dB is a poor figure wherever it turns up.
For audio it is, where 60 dB counts as quiet and 90 dB as excellent. For a Wi-Fi link, 20 dB already carries data reliably.
Reference table
| Signal, noise | What it sounds like | SNR (dB) |
|---|---|---|
| 0.1, 1 | Buried in the noise | -20.00 |
| 1, 1 | Signal equals noise | 0.00 |
| 10, 1 | Usable, audibly noisy | 20.00 |
| 2, 0.02 | Clean line | 40.00 |
| 1, 0.001 | Studio quiet | 60.00 |
Questions
Is it 20 log or 10 log?
20 for amplitudes such as voltages, 10 for powers such as watts. Power goes with the square of amplitude, and squaring inside a logarithm doubles the factor in front.
What is a good signal-to-noise ratio?
For audio, 60 dB and above is quiet and 90 dB is excellent. For Wi-Fi, anything from 20 dB up carries data reliably.
Can the ratio be negative?
Yes, and it means the noise is louder than the signal. Modern radio links still decode below 0 dB by spreading each bit over a longer time.
Does the unit of the amplitudes matter?
No, as long as both use the same one. The division cancels the unit, which is why the answer is a pure decibel figure.
Sources and last check
Information, not professional advice.